1. Field of the Invention
The present invention relates generally to a spectrum analyzer which can detect spectrum values at respective frequencies of a signal to be measured ("object signal" hereinafter) and can display the detected spectrum values in two-dimensional manner, and, more particularly, to a spectrum analyzer that can be used for measuring EMI (electromagnetic interference).
2. Description of the Related Art
In general, a spectrum analyzer is used as an apparatus for examining which kind of frequency component is included in a signal having a wide frequency band, which is received by a receiver in a radio monitoring apparatus. As is well known, in this type of spectrum analyzer, spectral values (amplitudes) at respective frequencies of an object signal are displayed, for example, on a CRT display, with frequencies being set in the abscissa.
A monitoring person finds, from the spectrum characteristics displayed on a CRT display, the signal level and frequency of the peak value of the amplitude of measured frequency component, and specifies the radio intensity and frequency corresponding to the peak value. In this way, the radio intensity is found from the peak value of the amplitude of the measured frequency component.
This technique is sufficient to examine general radio intensity. However, in the case of so-called electromagnetic interference (EMI) measurement for evaluating interference radio or noise, it is necessary to determine, in advance, conditions for finding spectrum values at respective displayed frequencies. As one of these conditions, the regulations of CISPR (International Special Committee on Radio Interference) mentions the use of a QP (quasi-peak) detector circuit having QP detection characteristics. The QP detection circuit includes a kind of time-constant circuit, in order to eliminate an instantaneous peak value due to noise, etc. from the found spectrum value.
FIG. 5 shows a structure of this type of spectrum analyzer having the QP detector circuit. An object signal a input from an input terminal 1 is supplied to a mixer 2. On the other hand, a local oscillation frequency signal b is supplied from a local oscillator 3 to the mixer 2. The object signal a supplied to the mixer 2 is frequency-converted to an intermediate frequency (IF) signal c. The IF signal c is fed to a band-pass filter (BPF) 4 and its frequency band to be passed is limited. An output of the BPF 4 is supplied through a change-over switch 5a to a peak detector circuit 6 or to a QP detector circuit 7 serving as a measuring detector circuit. A signal detected by either peak detector 6 or QP detector 7 is supplied to a signal input terminal Y of a CRT display 8 through a change-over switch 5b cooperating with the change-over switch 5a, as a spectrum value signal d at a frequency designated by the local oscillator 3.
The frequency of the local oscillation frequency signal supplied from the local oscillator 3 to the mixer 2 is varied (or swept), depending on the voltage value of a sawtooth signal e output from a sawtooth signal generator 9 with a cycle T. The sawtooth signal e is also supplied, as a sweep signal, to a sweep terminal X of the CRT display 8.
The peak detector 6 does not include a time-constant circuit which is provided in the QP detector 7. Thus, the peak detector 6 outputs, as spectrum value signal d, the peak value of the IF signal fed from the BPF 4, that is, the peak value of the magnitude of the spectrum analyzed by the BPF 4.
In the spectrum analyzer having the above structure, the sawtooth signal e with constant cycle T is supplied from the sawtooth signal generator 9 to the local oscillator 3 and to the sweep terminal X of the CRT display 8. Thus, when the change-over switches 5a and 5b are connected to the peak detector 6, spectrum values corresponding to respective frequencies are displayed on the CRT display 8, as shown in FIG. 5, with the frequencies plotted in the abscissa. In other words, with the use of the peak detector 6, sharp peak waveforms 11 appear in spectrum characteristic 10, as shown in FIG. 5.
On the other hand, when the change-over switches 5a and 5b are connected to the QP detector 7, the sharp peak waveforms 11 are lessened by the time-constant circuit built in the QP detector 7.
The problems of the spectrum analyzer shown in FIG. 5 will now be described.
FIG. 6 illustrates how the peak detection output and QP detection output are different when a pulse signal is input and conventional peak detector 6 and QP detector 7 (CISPR standard) are employed. As shown in FIG. 6A, a pulse signal having a relatively small pulse width t1 is input as object signal a. In this case, a level VP of the peak detection output (shown in FIG. 6C) obtained by peak-detecting the IF output signal (shown in FIG. 6B) differs largely from a level VQ of the QP detection output (shown in FIG. 6D) which is obtained by QP-detecting the IF output signal (FIG. 6B). In contrast, when a pulse signal having a relatively large pulse width t2 is input as object signal a, as shown in FIG. 7A, a level VP of the peak detection output (FIG. 7C) obtained by peak-detecting the IF output signal (FIG. 7B) becomes substantially equal to a level VQ of the QP detection output (FIG. 7D) obtained by QP-detecting the IF output signal (FIG. 7B).
When the object signal a is measured through the mixer 2 and BPF 4, as in the spectrum analyzer shown in FIG. 5, if the object signal a is a pulse signal with small width and low frequency, the BPF 4 outputs that portion of the high frequency component of the pulse waveform, which is allowed to pass through the BPF 4, during a time period corresponding to the pulse width of the pulse waveform. Thus, the level of the signal output from the QP detector 7 becomes lower as the object signal a has smaller width and lower frequency. For this reason, a dynamic range (or an overload coefficient) required for the QP detector 7 increases. In other words, in the case of the QP detection, the output level varies in accordance with the ratio (time ratio) of time occupied by the object signal; thus, it is necessary to increase the dynamic range.
In the case of EMI measurement using a spectrum analyzer, a high frequency component fx of an object signal (basic wave f0) is often measured selectively as shown in FIG. 8.
Since the waveform of the object signal a is not predictable, it is impossible to find, in advance, a signal level Vo of the object signal a only from the QP-detected spectrum value. Thus, under the CISPR regulations, it is decided that in the case of QP detection the input signal level of the QP detector 7 is made lower than a reference level by 43.5 dB (overload coefficient), as shown in FIG. 9, in order to prevent saturation of the QP detector 7 and its peripheral circuits.
However, when the dynamic range for analysis of the QP detector 7 is, for example, only 45 dB, the actual measurement dynamic range for a pulse signal with small pulse width is reduced to 1.5 dB (excluding the overload coefficient), and the precision in measurement is lowered.
Under the circumstances, it is possible to ignore the aforementioned regulations and to increase the input signal level of the QP detector 7 by 10 dB, whereby the input signal level is set to a value lower than the reference level by 33.5 dB and the measurement dynamic range for the pulse signal is increased to 11.5 dB.
In this case, however, the pulse signal with small pulse width is input and it is difficult to check whether or not the QP detector 7 is saturated.
This being the case, it may be thought that, as shown in FIG. 5, the peak detector 6 is provided in addition to the QP detector 7 serving as the measurement detector. First, the spectrum characteristic 10 is displayed on the CRT display 8 by using the peak detector 6, thereby confirming that no saturation occurs at a tip portion of each peak waveform 11. Thereafter, the change-over switches 5a and 5b are connected the QP detector 7 to perform regular measurement.
However, when the detectors 6 and 7 are changed over by means of change-over switches 5a and 5b, the measurement signal of the spectrum value obtained by the peak detector 6 and the measurement signal of the spectrum value obtained by the QP detector 7 are not considered to be identical, in strict sense, because there is a time difference therebetween. It is, therefore, impossible to confirm that the measurement signal corresponding to each spectrum value detected by the QP detector is not actually saturated in the QP detector.
It should be noted that in this specification the term "spectrum value" means not a frequency value but an amplitude value of a spectrum having a given frequency.
In brief, in the conventional spectrum analyzer, the EMI measurement is carried out by using the QP detector. The QP detector, however, has only a dynamic range of 45 dB, despite the fact that it is necessary to set a reference level to a value higher than the received signal by 43.5 dB corresponding to an overload coefficient (a margin for a pulse input signal). Because of this, a substantial dynamic range is only 1.5 dB, and it is difficult to perform a wide-band measurement with use of the same reference level. In addition, in the case of a narrow-band measurement, the QP measurement can be performed, only after the received level of the received signal is precisely measured, and the reference level is set within 1.5 dB. Consequently, it is difficult to precisely measure signals having low repeatability.